Description of the Related Art
There is demand to make mobile devices such as cell phones and PDAs thinner, even as thin as a credit card. Mobile devices may include conventional small cameras. However, due to limitations of conventional camera technology, small cameras used in such devices tend to capture images at lower resolutions and/or with lower image quality than can be achieved with larger, higher quality cameras. Making these conventional cameras thinner to work in thinner mobile devices tends to further degrade the quality and resolution of images that can be captured with conventional camera technology. Thus, there is a need for camera technology that may be integrated in thin devices that, while small and thin, capture higher resolution, higher quality images. However, for optical reasons, cameras with F/numbers lower than 1 are very hard to achieve. (The F/number is defined as the distance from the main lens to the photosensitive surface divided by the aperture of the main lens.) Practically, cameras may be limited to F/numbers not much lower than 2. With the practical limitation that the F/number needs to be about 2 or larger, such a camera would need to be small and thin (5 mm or less). A problem is that conventional small cameras tend to employ small pixels to achieve the same pixel count as in larger cameras. However, pixels cannot be smaller than the wavelength of light (˜500 nanometers) or the diffraction limit of the main lens of the camera. These thresholds on pixel size, along with other factors, have limited attempts to shrink the conventional camera while still providing images of sufficient quality and resolution.
Plenoptic Cameras
In contrast to conventional cameras, plenoptic, or radiance capturing, cameras sample the four-dimensional (4-D) optical phase space, and in doing so capture information about the directional distribution of the light rays. This information captured by plenoptic cameras may be referred to as the light-field, the plenoptic function, or radiance. In computational photography, a light-field (which may also be referred to as radiance) is a 4-D record of all light rays in 3-D. Radiance describes both spatial and angular information, and is defined as density of energy per unit of area per unit of stereo angle (in radians). A plenoptic camera captures radiance in plenoptic images (also referred to as flat images, or flats). When processed, plenoptic images may be digitally refocused, noise may be reduced, viewpoints may be changed, and other plenoptic effects may be achieved. Note that, in the literature, plenoptic cameras may also be referred to as light-field cameras, and plenoptic images may also be referred to as light-field images.
The light-field is the radiance density function describing the flow of energy along all rays in three-dimensional (3D) space. Since the description of a ray's position and orientation requires four parameters (e.g., two-dimensional positional information and two-dimensional angular information), the radiance is a four-dimensional (4D) function. This function may be referred to as the plenoptic function. Image photosensor technology, on the other hand, is only two-dimensional, and 4D radiance must therefore be captured and represented in flat (two dimensional) form. A variety of techniques have been developed to transform and capture the 4D radiance in a manner compatible with 2D photosensor technology. This may be referred to as a flat representation of the 4D radiance (or light-field), or simply as a flat.